Effect of cohesive energy on thermal expansion behavior of T6 tempered 7A09 aluminum alloy

The dynamic effect of cohesive energy on the CTE of T6 (the highest strength temper) tempered 7A09 aluminum alloy was studied in this paper. Transmission electron microscopy (TEM) was combined with differential scanning calorimetry (DSC) to analyze the phase transitions of T6 tempered 7A09 aluminum alloy taking place in a heating process. The corresponding CTE curve was recorded by a thermal dilatometer. The ion-bonding model was utilized to estimate the variation of cohesive energy caused by the phase transitions. Results indicate that GPII zones and η′ phase are main precipitates in T6 tempered 7A09 alloy. During the heating process, the cohesive energy of alloy system increases with the transition from η′ to η phase and the precipitation of η phase. The dissolution of η phase leads to a decrease of cohesive energy. Although volume changes of alloy system have effects on the CTE, the effect of the cohesive energy change is more obvious than that of volume change after the precipitation of η phase begins in the Ostwald ripening stage. Effects of cohesive energy and volume changes can be expressed in the nonlinearity of CTE curve.

► Transformation related to η’ and η can cause cohesive energy and volume changes.
► These changes can influence the variation trend of CTE for 7A09 alloy.
► Cohesive energy change becomes a main factor influencing CTE from Ostwald ripening stage.